In many applications, particularly for mobile hand-held devices, solid-state power amplifiers are typically the most costly component and are the largest user of electrical power of all components in such devices. Mobile hand-held devices typically utilize batteries, with battery life a significant concern in wireless communications devices such as cellular telephones, pagers, wireless modems, laptop computers with wireless capability, etc. Radio-frequency transmission, especially, consumes considerable power. A contributing factor to such power consumption is inefficient power amplifier operation. A typical RF power amplifier for wireless communications operates with only about 10% efficiency. Clearly, a low-cost technique for significantly boosting amplifier efficiency is needed.
To accommodate the power consumed by such power amplifiers the batteries utilized by such handheld devices are relatively heavy, expensive and occupy a fair amount of space in the hand-held devices. There is always a design tradeoff between the weight and size of a battery and the operational life of the battery. Increasing the efficiency of power amplifiers used in hand-held devices reduces the amount of power consumed, and can therefore decrease battery size and weight, or increase device operating life.
Current low-voltage power amplifiers may lose efficiency over temperature variations and drain voltage variations (between about 2-4V). Increased battery power consumption may results from this lack of efficiency. Current low-voltage power amplifiers may also have limited bandwidth coverage. A need therefore exists in the art for a compact, reliable, high-efficiency power amplifier for operating at low voltages, and over approximately one octave bandwidth.
As is known, pseudomorphic high electron mobility transistors (PHEMTs) are extensively used in wireless communication systems for switching, power and low noise amplifier applications. These transistors find wide market acceptance because of their high RF gain and power added efficiency (PAE), low noise figure (NF) and high reliability. The excellent properties of these transistors also make them attractive for use in satellite communication systems including direct broadcast satellite television (DBS-TV) and global satellite communication systems. PHEMT technology is also used in high-speed analog and digital IC's such as 2.5-10 Gb/s light wave communication systems. The higher frequency response of PHEMTs are currently finding use in millimeter wave communications (40 Gb/s) and radar systems.
In the prior art, to increase the RF power that a PHEMT transistor amplifier can supply, the width of the gate finger of the transistor is increased. However, the bandwidth of the transistor is reduced as the gate width is increased. To adjust for this decreased bandwidth, in the prior art a transistor with a shorter gate finger width is utilized since the frequency response of the transistor is inversely proportional to the gate finger width. The number of individual transistor cells connected in parallel, which form the total gate periphery, is increased. However, this increases the physical size of the PHEMT transistor's drain manifold, which decreases the frequency response. In addition, prior art PHEMT power transistors operate at relatively high voltages.
Thus, there is a need in the prior art for a PHEMT transistor power amplifier that can deliver increased power over a broad bandwidth but being smaller than equivalent prior art PHEMT transistor power amplifiers, and having high-efficiency, being very reliable and capable of operation at low voltages. Further, there is a need for a PHEMT transistor power amplifier that can operate at low voltages on the order of 3.3 volts, which matches the voltage of many current batteries, and therefore reduces battery weight compared to prior art battery powered PHEMT transistor power amplifiers. In addition, there is a need for a PHEMT transistor power amplifier that has improved power handling capability in a small size while operating at lower temperatures due to the increased efficiency of the transistor.